What Is a Complete Protein—And Does Your Body Actually Know the Difference?

Caroline Tien
·9 min read

Tanja Ivanova/Getty Images

When you first hear the term “complete protein,” you may just think it’s one of those marketing buzzwords used to make products sound, well, healthier (see also: “all-natural”). But it’s actually a legit nutrition term—even if the meaning isn’t immediately clear.

Before we delve into what a complete protein actually is, let’s take a step back for a sec and talk about why all types of protein are important. One of the three macronutrients—along with carbs and fats—protein famously offers a post-workout boost, but that’s not all. “Protein’s important for more than just muscle building,” Desiree Nielsen, RD, a recipe developer with a focus on plant-based nutrition, tells SELF. Not only does it fill you up, she says, “it helps to keep blood sugar balanced, it’s important for bone health, and it’s important for immune function.” As for how much you need to reap these benefits? Sources differ, but Harvard Health reports that men and women should generally aim to consume about 46 and 56 grams per day, respectively (or, if you’re willing to do some math, about 0.36 grams per pound of body weight). If you exercise, though, that number climbs higher: The American College of Sports Medicine recommends active folks (like those who lift weights or are training for a running or cycling event) take in 0.5 to 0.8 grams per pound.

Okay, protein is super important, but how does a complete one fit into the picture? Read on to find out what a complete protein actually is, why they’re important, and the best way to fit them into your diet.

What is a complete protein, anyway?

To answer this question, let’s think back to high school bio class. You might dimly remember your teacher mentioning amino acids—molecules that combine to form proteins, otherwise known as the building blocks of life. There are a total of 20 amino acids, and they are divided into three main categories: essential, semi-essential (or conditionally essential), and non-essential.

“Non-essential means that our body can make them itself, so there’s no requirement to get them directly from food,” Nielsen says. Semi-essential amino acids are also synthesized by your body, but need to be supplemented during times of stress. Finally, “essential means the body cannot produce them at all, and it is imperative that we get them from food.”

Of the 20 amino acids, nine are considered essential: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Basically, complete proteins are foods that contain all nine in adequate amounts, and this includes most animal products, Janice Dada, MPH, RDN, a certified intuitive eating counselor based in California, tells SELF.

Both animal and plant proteins contain some combination of the 20 amino acids, but not necessarily all nine essential ones. In contrast to complete proteins, those that don’t contain all nine in adequate amounts are known as incomplete proteins.

Most plant products fall into this category, including beans, grains, nuts, peas, seeds, and vegetables. Usually, they’re considered incomplete proteins because they’re deficient in one or two of the essential amino acids—“what we call limiting amino acids,” Nielsen says.

Not all incomplete proteins have the same limiting amino acid or acids. For grains, it’s typically the amino acid lysine, while for legumes like lentils and beans, it’s usually methionine or tryptophan—and remember that, because it’ll come into play later.

What makes complete proteins so important?

Essential amino acids aren’t labeled “essential” for nothing: Without an external supply, your body wouldn’t be able to function correctly, and you wouldn’t be able to survive.

“When we digest the protein we eat in food, we break it down into amino acids, and then our body does different things with it—makes antibodies, makes muscles, makes skin cells, makes hair. And to make those different things, it needs different combinations of amino acids,” Dada says. Meaning: If you’re deficient in any of those amino acids, your body’s ability to carry out all these tasks is going to take a hit.

So complete proteins are important because they’re an efficient delivery vehicle for the nine essential amino acids—the idea being that “if that’s all we could eat every day, a complete protein would ensure we don’t die from essential amino acid deficiency,” Nielsen says. Whereas an incomplete protein like wheat (or another grain) “would not sustain you, because you’d be low in lysine,” she adds.

While protein deficiency is rare in the West, it affects millions of people worldwide, especially in developing countries. Symptoms of a severe deficiency include brittle hair and nails, frequent bouts of illness, mood changes and trouble thinking, and hunger and weakness. Globally, 212,242 deaths occurred due to protein malnutrition in 2019, according to a study published in the December 2023 issue of Public Health.

But even if you’re not at risk of an actual deficiency, you still shouldn’t discount the importance of taking in enough protein—not only for your physical health, but also your overall well-being. We’ve already mentioned that protein can help keep you satisfied post-meal, but it can also strengthen your bones, build muscle, speed up recovery, and prevent injury—which, in turn, can help you meet your fitness goals (and simply feel pretty damn good each day too).

And we should also point out that certain demographics have higher protein intake needs, including babies, children, pregnant and lactating folks, and older people. In fact, women over age 65 who suffer from decreased bone density may need as much as 1.3 grams per pound to prevent or treat fractures.

What foods count as complete proteins?

Thankfully, complete proteins aren’t hard to come by in your diet—and, yes, there are even some plant-based sources for all the vegans and vegetarians wondering.

  • Meat (like beef and pork)

  • Poultry (like chicken and turkey)

  • Seafood (like fish and shellfish)

  • Eggs

  • Dairy products (like milk, cheese, and yogurt)

  • Soy (like tofu, edamame, tempeh, and miso)

  • Quinoa

  • Buckwheat

  • Amaranth

  • Hemp seeds

  • Chia seeds

  • Blue-green algae (like the smoothie add-in spirulina)

  • Nutritional yeast (also known as “nooch”)

Just don’t confuse a complete protein for a major source of protein—as in, one that packs a ton of the stuff. Quinoa is a prime example, according to Nielsen. “Quinoa may have a balanced amino acid profile, but it’s not high in protein,” she says—only around eight grams per cup. But it’s also important to remember that protein is far from the only nutrient that counts. Take hemp hearts (the soft, chewy innards of hemp seeds), for instance. On top of their protein content, they’ll also provide “your daily requirement of omega-3s along with a host of other vitamins and minerals, like zinc and iron,” Nielsen points out. Micronutrients like these are also critical to keeping you healthy and happy, so they should factor into your food-related decision-making too.

What if I’m not feeling any of these options?

Not to worry. “You don’t need to get all nine essential amino acids in a single food, nor do you need to get them even in perfect amounts at a single meal,” Nielsen says. “Biologically, a ‘complete protein’ is actually unnecessary.” Instead, it’s more about the cumulative effect: “We just want to get a range of different amino acids throughout the day,” Dada says. TL;DR: Yes, you want to take in the full complement of amino acids, but you don’t have to do it all in one shot by eating a complete protein!

Remember what we said earlier about how incomplete proteins aren’t necessarily deficient in the same amino acid or acids? Well, our muscles can’t tell the difference between a complete protein and a bunch of incomplete proteins that complement each other, so you can use that information to your advantage by mixing and matching incomplete proteins to fill the gaps.

Say you have two incomplete proteins in front of you: one that’s deficient in lysine but contains plenty of methionine (say, a grain like rice), and one that lacks methionine but packs lots of lysine (say, beans). Since they each provide an amino acid that the other one skimps on, pairing them together—think a simple bowl of beans and rice—will meet your essential amino acid needs. “Independently, they’re incomplete, but together, they are complete,” Dada says.

Here’s some recommended incomplete protein combos that amount to a complete protein:

  • Grains and legumes

  • Legumes and nuts or seeds

  • Vegetables and grains, nuts, or seeds

And some examples of how to translate that to your plate:

  • Peanut butter sandwich

  • Oatmeal topped with peanut butter

  • Bean taco or burrito

  • Hummus and pita

  • Seitan and soy sauce

  • Salad with chickpeas and sunflower seeds

And in fact, deliberate mixing and matching might not even be necessary.

Once upon a time, experts thought that you had to pair complementary incomplete proteins together (as in, eat them at the same time) to reap the benefits of a complete protein, but nowadays that line of thinking has fallen out of favor. Just try to eat them within the same day. Eating beans at one meal and then snacking on some raw almonds later in the day will do the trick, for example.

And assuming you stick to a varied diet, you probably don’t even need to make a conscious effort to pair complementary incomplete proteins, according to Dada. Sure, if you’re only eating beans all day every day, “that could be somewhat problematic,” she says, but if your daily meals contain some diversity, “it’s likely that you’re getting complete protein” already. Yep, even for strictly plant-based folks: Just a standard mix of beans, soy-based foods, and different seeds and grains “is going to give them what they need,” Dada says.

Unless your protein intake is very narrow (restricted to a single food or a few foods), you should be A-OK. “The big take-home is that what is more important than eating complete proteins, is eating enough protein over the course of the day from a variety of foods,” Nielsen says. And if you need help doing that, check out this list of high-protein vegetables that will flesh out any side or salad, this roundup of high-protein snack ideas that’ll help you beat those post-lunch cravings, or this explainer on how to turn avocado toast into a high-protein breakfast.

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Originally Appeared on Self

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